This invention relates to a conductivity modulation type Metal-Oxide Semiconductor Field Effect Transistor ("MOSFET") and, in particular, to a conductivity modulation type MOSFET which has no negative resistance in the operating range of its I-V characteristic.
FIG. 2 represents an example of a conventional conductivity modulation type MOSFET The following description assumes that the first conductive type is an N-type and the second conductive type is a P-type. Typically, the MOSFET has a P-type silicon substrate 2 formed by epitaxial growth of an N.sup.- type buffer layer 3 and an N-type high resistance layer 4. Next, a P.sup.+ type low resistance layer 7, a P-type base layer 5 and an N.sup.+ type source region 8 are formed by means of diffusion. Part of an insulating layer 9, formed on top of the silicon substrate, has a gate layer 10 on top of it. The rest of the insulating layer 9 is formed over the gate layer 10. Formed over the insulating layer are a source electrode 11 and a drain electrode 12. In such a conventional type MOSFET, the impurity density of the N.sup.+ buffer layer 3 typically exceeds 10.sup.17 cm.sup.-3 in order to prevent punch-through caused by the elongation of a depletion layer at the time of forward voltage. This also acts to shorten the switching time while maintaining the ON resistance at a low value.
When a thick N-type high resistance layer is used to allow the FET to block a high voltage and when electron beam irradiation or gold or platinum diffusion is used to shorten the switching time, the resulting current-voltage characteristic has a negative resistance area. This characteristic is represented by curve B in FIG. 3. This phenomenon makes the MOSFET significantly more difficult to use than a device with a normal I-V characteristic. Curve A represents a normal I-V characteristic.